1. Field of the Invention
The present invention relates to a rotor blade for rotary wing aircraft such as a helicopter, and particularly to a rotor blade for rotary wing aircraft having a special blade tip planform shape.
2. Description of the Related Art
FIG. 8 is a view showing rotor aerodynamic environment of a helicopter in forward flight. As shown in FIG. 8A, when a helicopter 1 flying at forward speed V with a rotor having radius R which rotates at angular speed .OMEGA., the relative airspeed varies significantly between an advancing blade where the angular speed .OMEGA.R of the rotor is added to the forward speed V and a retreating blade where the forward speed V is subtracted from the angular speed .OMEGA.R of the rotor.
At a position where azimuth angle .PSI. (angle measured counterclockwise from the rearward direction of the helicopter 1) equals to 90.degree., the airspeed of the advancing blade reaches a maximum and the airspeed of the blade tip becomes .OMEGA.R+V. At a position of azimuth angle .PSI.=270.degree., on the other hand, the airspeed of the retreating blade reaches a minimum and the airspeed of the blade tip becomes .OMEGA.R-V. The airspeed of an intermediate portion of the blade takes a value obtained by proportional distribution of .OMEGA.R+V and .OMEGA.R-V. For example, when .OMEGA.R=795 km/h and V=278 km/h are assumed, the airspeed at a position of about 35% from the root end of the retreating blade becomes zero, as shown in FIG. 8A.
When a helicopter flies at high speed, in particular, the airspeed at a tip of an advancing blade reaches a transonic speed resulting in a strong shock wave. A noise generated by the strong shock wave is called high-speed impulsive noise. A phenomenon called delocalization in an ultrasonic region takes place at this time in a coordinate system viewed from the rotor blade which is in rotational motion. The shock wave generated is transmitted through the delocalized ultrasonic region over a great distance, making a high noise to be heard at a distance.
Since the airspeed of a retreating blade is significantly lowered, the angle of attack a of the blade must be greater in order to produce a lift similar to that of the advancing blade, and it is common to use a cyclic pitch control wherein the pitch angle of the blade is controlled in accordance to the azimuth angle .PSI.. While the pitch angle of the blade is controlled by means of sine wave of which amplitude is minimum at azimuth angle .PSI.=90.degree. and maximum at azimuth angle .PSI.=270.degree., the angle of attack .alpha. of the blade in this case varies in the direction of span as shown in FIG. 8B due to flapping of the blade itself. For example, when .PSI.=90.degree., the angle of attack .alpha. of the blade becomes about 0.degree. at the root end and about 4.degree. at the tip end. When .PSI.=270.degree., the angle of attack .alpha. of the blade becomes about 0.degree. at the root end and about 16 to 18.degree. at the tip end, thus exceeding the stalling angle. When the angle of attack .alpha. of the blade exceeds the stalling angle, lift coefficient Cl and pitching moment coefficient Cm change rapidly, causing to violent vibration of the helicopter structure and a high fatigue load being applied to the pitch link.
Design items used for evaluating the characteristics of an advancing blade include high-speed impulsive noise and those for evaluating a retreating blade include maximum lift coefficient Clmax and stalling angle. The maximum lift coefficient Clmax is defined as the maximum value of lift coefficient when the angle of attack .alpha. of a blade having a particular aerofoil section is just before the stalling angle. A blade is considered to be better blade when the high-speed impulsive noise and the absolute value of pitching moment coefficient Cm are smaller, and the values of the maximum lift coefficient Clmax and stalling angle are greater. Applying the blade tip portion with a sweptback angle is an example of reducing high-speed impulsive noises. The blade tip with the sweptback angle mitigates the shock wave and somewhat decreases the noises, though the delocalized supersonic region itself remains and the noise is still at a significant level. Moreover, in the case a large sweptback angle is given to the blade tip, blade tip stalling occurs at a smaller angle of attack, resulting in rapid change in the pitching moment coefficient Cm and a decrease in the maximum lift coefficient Clmax.